1. Field of the Invention
Embodiments relate to methods and devices for establishing a routing metric in mesh networks, in particular according to standard IEEE 802.11s for Wireless Mesh Networks, wherein the routing messages sent on various paths from the source node via the mesh nodes of the mesh network to the target node are interpreted in order to determine the best path for sending data packets from the source node to the target node.
2. Background of the Related Art
WO 2008/122674 A2 relates to a method for operating a mesh-type network, in particular according to standard IEEE 802.11s, wherein the network comprises multiple network nodes. From this it is known that six addresses are to be used in the data packets.
In the PCT application PCT/EP2009/003271 filed on May 7, 2009, a method and device for generating at least one extension of an allocation message for wireless mesh networks are described, in which the creation of possible allocation tables for data packets to be transmitted is described.
A fundamental principle of wireless mesh networks or mobile ad hoc networks is the forwarding of data packets from other nodes through the mesh nodes. This means that a mesh node also needs power for sending and receiving data packets, even if the node itself has nothing to send or receive. This can cause more rapid battery consumption by battery-operated mesh nodes.
To extend battery life, battery-operated devices often also use an energy-saving mode, which puts their wireless interfaces temporarily into a sleep mode. This sleep mode requires only a very small amount of power, but devices in sleep mode cannot send or receive data packets. There is no loss of data, because the data packets can be stored temporarily until the device in energy-saving mode comes out of its sleep mode. However, this increases the delay in transmitting the packet. Depending on the sleep mode duration, this can be a significant amount of time, especially with multi-hop connections through devices in energy-saving mode in wireless mesh networks or mobile ad hoc networks.
Standard IEEE 802.11s on WLAN Mesh Networking, which is currently being developed as Version “IEEE P802.11s™/D2.0 Draft STANDARD for Information Technology—Telecommunications and Information Exchange between Systems—Local and metropolitan area networks—Specific requirements, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications, Amendment <number>: Mesh Networking,” March 2008, pages 1-XVII and 1-242—hereinafter designated as “IEEE P802.11s/D2.0 Draft Standard”—describes in its Section 11 B.9, pages 181-206, the routing protocol “Hybrid Wireless Mesh Protocol (HWMP),” which includes a management variable that establishes whether or not a mesh node should forward data packets to other mesh nodes. If no data packets should be forwarded, the routing messages are processed by HWMP in such a way that no path can be created through these nodes. However, because such non-forwarding nodes can be only the end or start point of a path, the mesh network might break into two or more sections if that node is the only connection.
In the publication by Ian D. Chakeres, Elizabeth M. Belding-Royer: “Transparent Influence of Path Selection in Heterogeneous Ad hoc Networks,” 15th IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC), Barcelona, Spain, September 2004, 3 pages, a characteristic of the Ad hoc On-Demand Distance Vector (AODV) is used in selecting the path. The target node responds in Standard AODV [see also Charles E. Perkins, Elizabeth M. Belding-Royer, Samir R. Das: “Ad hoc On-Demand Distance Vector (AODV) Routing,” IETF RFC 3561 (Experimental), July 2003, pages 1-36] only to the first route request message; subsequent route request messages are ignored. For this reason, forwarding of route request messages to battery-operated mesh nodes is delayed, so that route request messages from paths without battery-operated interim mesh nodes or with the fewest battery-operated interim mesh nodes are received first by the target node. HWMP from IEEE P802.11s/D2.0 Draft Standard is indeed based on AODV, but it recognizes all route request messages to the target mesh node. This mechanism therefore cannot be used in an IEEE P802.11s/D2.0 Draft-Standard mesh network with HWMP.
In the publication by Michelle Gong, Kazuyuki Sakoda, Jarkko Kneckt: “Thoughts on Interaction between Power Management and Path Selection,” July 2007, IEEE 802.11, Document 11-07/2095r3, pages 1-23 and in particular pages 17-18, two ideas are mentioned regarding how paths through battery-operated mesh nodes can be avoided:                In the first idea, which is only roughly outlined, mesh nodes are classified depending on their power source (power cable or battery) and on the battery status (non-critical/critical). This classification is relayed by beacons, so that routing messages can be forwarded selectively.        In the second idea, a wake-up bit is attached to the routing messages. If it is not added, battery-operated mesh nodes ignore those routing messages. If it is added, those routing messages are processed through the battery-operated mesh nodes. If a node wants to create a path, initially the wake-up bit is not added, so that in the case of successful path creation, no battery-operated devices are included in that path. If no path can be created, the wake-up bit is added, so that now battery-operated devices can also be considered for creating the path.        
This method has three significant disadvantages:                The long wait time until a path is found, if only paths through battery-operated devices are available.        A path through multiple battery-operated mesh nodes with a very good path metric is preferred over a path with only one battery-operated mesh node but a somewhat poorer path metric, even if the poorer path is qualitatively satisfactory and has a better power balance.        The target mesh node receives no information about whether or not a path runs through battery-operated mesh nodes.        
In the publication by Liwen Chu, George Vlantis: “Symmetrical Airtime Link Metric Report and Path Loop Avoidance,” April 2008, IEEE 802.11 Document 11-08/0636r0, pages 1-10 and in particular pages 3-4, it is proposed that battery-operated mesh nodes add the length of the beacon interval divided by 2 to the link metric. This proposal was made specifically for the airtime link metric included in the IEEE P802.11s/D2.0 Draft-Standard. It means that battery-operated mesh nodes are rarely selected, but the target mesh node receives no information about whether or not a path runs through battery-operated mesh nodes.